It has been known in the art to plate metals upon a substrate, usually itself metal, by applying to plating metals, in particulate form, mechanical forces sufficient to produce such a close association of the plating metal particles and the surface of the article to be plated that adhesion occurs between the article surface and the plating metal particles. To produce the mechanical forces necessary to provide such adhesion, typically the plating metal particles, a solid impacting media and materials which have been found to be effective in the promoting of such plating are placed in a ball mill or tumbling barrel and rotated together. In this manner, the kinetic energy of the moving impacting media is transferred to the plating metal particles in a fashion such that these particles are hammered or pounded by the impacting media onto the surfaces of the articles to be plated.
The initial work in this field of mechanical plating, as described above, has been undertaken by Clayton and Pottberg, such work being discussed in U.S. Pat. Nos. 2,640,001; 2,640,002; Re. 23,861; 2,689,808; 2,723,204, etc. Typically, the aforementioned processes are undertaken in the presence of a liquid, and the agitated conglomerate or slurry contains what are commonly referred to as "promoter chemicals". Such promoter chemicals typically comprise unsaturated fatty acids and the like, film-forming materials and surfactants.
Further patents, such as U.S. Pat. No. 3,460,977, discuss the use of other promoter chemicals for effectuating the mechanical plating discussed above. In U.S. Pat. No. 3,328,197, there is taught a further improvement wherein the promoter chemical is provided in a solid cake or bar which contains a predetermined desirable combination of mechanical plating promoter chemicals. As the mechanical plating cycle progresses, the bar or cake dissolves at a rate which provides sufficient promoter chemical to optimize the mechanical plating process.
In U.S. Pat. No. 3,268,356 there is disclosed another refinement or improvement, wherein the promoter chemical and/or the plating metal particles are added to the plating barrel incrementally in successive additions, which is taught to optimize the density and uniformity of the plating over the entire surface of the article being plated.
A substantial breakthrough in mechanical plating is disclosed in U.S. Pat. No. 3,531,315, where it is disclosed that a composition containing a strong acid could be utilized during the mechanical plating process, thereby eliminating a costly and time consuming rinsing step which had been necessary prior to that discovery. In essence, the strong acid employed for cleaning and scale removal of the articles to be plated was also utilized during the mechanical plating process without necessity of an intervening rinsing step.
Gradually, it became desirable to increase the thickness and coating weight of mechanically applied metallic coatings, e.g., up to from about 0.7 to 2.5 ounces per square foot coating weight (1.5 to 5.3 mils coating thickness). During the development of such heavy coatings, it was found that the teachings of U.S. Pat. No. 3,268,356 were applicable in that incremental additions of metal powder enhanced the adhesion of plated deposits. Therefore, the commonly utilized citric acid-based chemistry, such as, for example, is discussed in U.S. Pat. No. 3,328,197, was amenable for use therewith. The operating pH of about 3.0 to 3.5 with this chemistry is less aggressive upon the metal powder, and also, the promoter chemicals could be introduced in bar form which slowly disintegrated during the process, thereby releasing the chemicals gradually as plating progressed. However, the relative high cost of organic acids and their salts, together with their known tendency to complex heavy metal ions to preclude or hamper effective effluent treatment made their use less than desirable economically.
Even though this system would function to provide heavy coatings on articles, it was therefore still desirable to optimize processing time in accordance with the teachings of U.S. Pat. No. 3,531,315, i.e., wherein a separate cleaning and rinsing step could be eliminated. However, the commercially available chemistry normally utilized with the process of the patent is not amenable to incremental metallic powder additions, as was the citric acid-based system, because the operating pH of this system, being in the range of 0.5 to 1.5, is typically too aggressive upon the metal powder. Furthermore, in this latter process, promoter chemicals are typically introduced in powder form at the start of the plating cycle with no intervening additions during the process. Typically, therefore, during a plating cycle utilizing this promoter chemistry in conjunction with incremental metallic powder additions, the metallic coating may not be deposited in a controlled fashion since the proper chemical environment would not be continually present during the entire plating cycle. Consequently, the conditions necessary to apply successive layers of well consolidated, adherent particlss could not be uniformly maintained.
While it has been desirable to utilize the processing techniques of U.S. Pat. No. 3,531,315 because such are typically based on a non-chelating and more economical acid system, such has not heretofore been available for the production of higher coating weights. However, I have now found a chemical composition system which is more amenable to utilization with incremental metal powder additions during the mechanical plating of higher coating weights. The system is utilized at the initiation of the plating cycle as well as during the plating cycle by incremental addition in conjunction with the metal powder.